1. Field of the Invention
This invention relates to an apparatus for decomposing waste plastic materials, such as cable sheathings, through a supercritical water reaction, so as to obtain oily materials from the reaction products.
2. Description of the Prior Art
A hydrothermal reaction in which high temperature, high pressure water and raw material are contacted and mixed with each other, or a supercritical fluid reaction in which a high temperature, high pressure supercritical fluid is used as a solvent is conventionally used in order to perform decomposition of various waste materials, synthesis of organic and inorganic compounds, production of particulates, and reaction between particulates. Raw materials to be treated include coal, heavy oil, rubber, waste plastic, excrement, PCB (polychlorobiphenyl). It is also proposed that particulates may be produced through a chemical reaction. It is noted, however, that such proposals are based on a laboratory scale. Thus, it is highly desired that a continuous treatment apparatus in a practical size be embodied which is indispensable to treatment of a large amount of raw material and industrialization.
A typical high-pressure treatment apparatus in prior art is described in Japanese Patent Publication No. 5-177188 (KOKAI HE15-177188). In this apparatus, material to be decomposed (object material) such as PCB, water or hydrothermal solvent, and reaction accelerator are contained in a decomposition reactor (autoclave) and then decomposed under a high temperature/high pressure condition. The above apparatus is of a batch type. Such apparatus includes, as shown in FIG. 1, a decomposition reactor (autoclave) 1. The decomposition reactor 1 includes a nitrogen gas cylinder 3 as pressure gas supply means, a pressure means 2 consisting of the nitrogen gas cylinder 3, and a heating means 4 such as electric heaters. Material to be decomposed (object material), water or hydrothermal solvent (solvent) 11 and reaction accelerator 12 in a given ratio are supplied to the decomposition reactor 1 through pipings 5, 6, 7 and pumps 8, 9, 10. The internal pressure within the decomposition reactor 1 is increased by means of the pressure means 2. Thereafter, the temperature in the reactor is adjusted at a selected value by means of the heating means 4. The object material is decomposed by maintaining the reactor at a high temperature/high pressure condition for a predetermined period of time. Then, the temperature in the reactor is decreased by de-energizing the heating means, while the internal pressure is decreased by opening a blow-off valve of the reactor. Then, a valve 14 of a decomposed liquid discharge means 13 is opened, and the reactor is pressurized by means of the pressure means 2, whereby a decomposed liquid is discharged into a decomposed liquid discharge tank 16. When solid materials are contained in the decomposed liquid, they are removed by means of a filter 17. If it is intended for the apparatus to be operated in a continuous manner, raw liquid or feed liquid should be continuously supplied to the reactor 1 and decomposed liquid should be continuously discharged from the reactor in order to maintain liquid level in the reactor at a predetermined value. It is noted, however, that, in such a case, the decomposed liquid essentially contains therein a portion of the raw liquid when it is discharged from the reactor. Thus, only insufficient treatment is expected when such prior art apparatus is operated in a continuous manner.
Japanese Patent Public Disclosure No. 4-2844886 (KOKAI HE15-284886) proposed a system in which feed material is supplied by means of a high-pressure injection pump into a vertical reaction tower consisting of straight pipes for continuous treatment thereof. In this system, water in a water tank 20 and feed material in a feed liquid tank 18 are displaced by means of a compressor 19 through a piping 21 into a reaction tower 29 consisting of a straight pipe (reaction pipe 30A) of a high pressure-resistance having heating means thereon 30, as shown in FIG. 2. Before reaching the reaction tower 29, the water and the feed material are admixed (dilution of the feed material) by connecting pipings 22 and 23, and introduced through a piping 24 into a heat exchanger 28 having a high-pressure injection pipe 25, piping 26 and piping 27. Treated liquid is discharged from piping 30B connected to the outlet port of the reaction tower through the heat exchanger 28, a cooler 30C and a pressure reducing valve 30D. In this continuously treating apparatus, various pipings, i.e., the piping for feeding the material to be decomposed (object material), the piping for supplying reaction accelerator, the heat-exchanging pipings disposed to upstream and downstream of the reaction tower, the vertical straight piping within the reaction tower, and the piping for discharging the treated liquid are employed. Thus, the entire flow passage is very complicated, though reactant liquid (liquid to be reacted) may flow through the straight piping within the reaction tower simply in the vertical direction.
In order to perform decomposition by means of a supercritical water reaction in a stable and efficient manner, it is necessary for object material to be treated and water to be sufficiently admixed and contact with each other and such well admixed and contacted condition should be maintained. It is also necessary for reaction temperature, pressure and treatment capacity (flow rate) of reactant material to be kept at a constant value or to be adjustably controlled. Take, for example, the case of decomposing high-molecular weight compounds such as waste plastics. The rate of their decomposition which is started by the supply of sufficient thermal energy is so slower at temperatures below the supercritical level than at the supercritical temperature that they take unduly prolonged time, which is undesirable from a practical viewpoint since the throughput of the decomposing apparatus is very low. Hence, the reaction temperature must be at least equal to the supercritical level. It has recently been proposed to lower the decomposition temperature as by adding an oxidizer or other decomposition accelerators but this idea also is not satisfactory from a practical viewpoint since the decomposition accelerator will not only corrode the apparatus but also add to the operating cost. On the other hand, at the supercritical temperature and above, there is no need to use any decomposition accelerator and yet the high-molecular weight compounds can be decomposed within a few minutes.
Supercritical water will undergo considerable changes in physicochemical properties with varying pressure and below the supercritical pressure, the supercritical water will not be able to form a homogeneous phase with the decomposed oil which is the product of decomposition and no efficient reaction site is available to ensure a homogeneous reaction which is indispensable to the progress of consistent decomposition. Specifically, the decomposition reaction by means of a supercritical water reaction may be facilitated and increased in its efficiency by severely contacting object material and water with each other. If object material and water are admixed and contacted insufficiently, reaction efficiency becomes very low, so that intended treatment is not achieved, whereby such process could not be used practically.
When the object material to be decomposed is solid or powder, it is not completely decomposed under a certain treatment condition, thus remaining in the form of solid or powder. When water and solid are not sufficiently admixed, such water and solid or powder are separated into two phases or layers. Thus, heterogeneous field is created within the apparatus, so that stable reaction is not expected. Even when the object material is liquid, efficient reaction would not be expected, if the object material, solvent and reaction accelerator are not sufficiently admixed and pass through the reactor in separate form. When by-product in powder form is generated during decomposition process, there is no problem if such powder and fluid are evenly admixed and transported smoothly. It is noted, however, that problems may be caused if there is difference in moving velocity between power and fluid. Such problems may include blockage in the reaction system due to coagulation of powder therein or blockage in a pressure reducing valve due to passage of massive powder therethrough.
If the above prior art technique could be practically used, it is impossible to perform certain operation such as agitation within a large-sized reactor vessel of a batch type. Thus, it is not expected for water and object material to be decomposed to be sufficiently admixed. When decomposed liquid contains solids therein, it is necessary for the decomposed liquid and solids to be extracted from the reactor vessel for removing such solids by means of a filter. It is noted, however, that it is practically difficult to securely extract decomposed liquid and solids from the reactor vessel at high-temperature and high-pressure and to stably perform filtration operation by means of a filter under high-pressure condition.
Another problem relating to the batch type high-pressure treatment is operational problem included when setting reaction time, reaction temperature, treatment capacity (flow rate) and the like. With the batch type treatment, operational condition depends upon a given reactor vessel, so that a wide variety of operational condition could not be selected. For example, a temperature gradient within the reactor vessel is required to be variously controlled in accordance with a given material to be treated. It is noted, however, that, with the batch type treatment, the temperature gradient could not be variously controlled.
Polyethylene, frequently used as cable sheathings, may be very quickly decomposed in a supercritical water at about 500.degree. C. into low molecular compounds such as straight chain hydrocarbons, such as paraffin and olefin, and aromatic compounds and the mixtures thereof.
Paraffinic hydrocarbons are a class of straight-chained hydrocarbons that do not have any unsaturated bonds in the molecule and they may be represented as follows: EQU --C--C--C--C--C--C--C--C-- . . . C--C--C--C--C--C--C--C--
A paraffinic hydrocarbon having two carbon atoms is called ethane (C.sub.2 H.sub.6 ; gas), depending upon the number of carbon atoms they have, paraffinic hydrocarbons are respectively called propane (C.sub.4 H.sub.10 ; gas), n-hexane (C.sub.6 H.sub.14 ; liquid); n-octane (C.sub.8 H.sub.18 ; liquid) and n-decane (C.sub.10 H.sub.22 ; liquid); a compound having 20 carbon atoms is called n-eicosane (C.sub.20 H.sub.42 ; solid).
The longer the carbon chain, the higher the boiling point and the more fluid at ordinary temperatures. Therefore, decomposed oils containing more of the paraffinic hydrocarbons having a longer carbon chain are comparable to heavy oils whereas those which have a shorter carbon chain contain more of the gasoline and gaseous components.
Olefinic hydrocarbons are a class of straight-chained hydrocarbons that have unsaturated bonds in the molecule and they may be represented as follows: EQU --C.dbd.C--C--C--C--C--C--C . . . C--C--C--C--C--C--C--C--
An olefinic hydrocarbon having two carbon atoms is called ethylene (C.sub.2 H.sub.4 ; gas); depending on the number of carbon atoms they have, olefinic hydrocarbons are respectively called propene (C.sub.4 H.sub.8 ; gas), n-hexene (C.sub.6 H.sub.10 ; liquid), n-octene (C.sub.8 H.sub.16 ; liquid) and n-decene (C.sub.10 H.sub.22 ; liquid); a compound having 20 carbon atoms is called n-eicosene (C.sub.20 H.sub.40 ; solid). The above compounds such as paraffinic hydrocarbons and olefinic hydrocarbons may be modified into aromatic compounds by controlling temperature of such supercritical water and treatment time. In order to perform continuous decomposition of polyethylene at high speed and subsequent, continuous modification reaction process, it is necessary to continuously and selectively control reaction temperature and treatment time, to change the temperature at an appropriate time, and to selectively control the reaction. In order to permit various object materials to be decomposed, other than polyethylene, a supercritical water reaction should be realized in which treatment condition such as treatment time and temperature control may be variously changed.
In the above-mentioned continuous treatment process, the straight pipe of the reaction tower may have an inner diameter of about 20 mm and an entire length of about 2,890 mm, as exemplified in the prior art documents, and thus its volume is strictly restricted. Thus, it is difficult to variously select treatment capacity (flow rate) and reaction time. If the straight pipe is increased in its diameter in order to obtain higher treatment capacity, heat capacity of such pipe will be disadvantageously increased in proportion to its increased volume. Thus, it is difficult to obtain efficient heating and preservation of temperature. If the reaction tower is increased in its height, it is also difficult to preserve reaction temperature and temperature gradient.
The reaction apparatus is operated at high-pressure and high-temperature, so that it is very dangerous if any damage is caused thereto. In particular, any troubles leading to burst-out of liquid to be decomposed during treatment should be avoided. Connection portions or connectors requiring seal material which may invite such troubles should be obviated. In the above-mentioned high-pressure treatment apparatus, the reactor tower/reactor vessel includes high pressure piping which is different to the inflow piping for the material to be treated. This means that there is difference in inner diameter between the inflow piping for the material to be treated and the reaction tower or reactor vessel and the outflow piping. This causes change in flow velocity and intermittent flow within the flow line. By this, it is difficult to analyze or determine the phase of each of the hydrothermal solvent and the object material to be decomposed within the reactor tower or reactor vessel. This makes it difficult to appropriately design the apparatus.
This invention provides an apparatus for converting waste plastic into oil, utilizing decomposition reaction by means of a supercritical water reaction, which may be operated in a safe manner, and which ensures object material to be decomposed and water to be sufficiently and evenly admixed so as to obtain a continuous flow, while facilitating selective setting and preservation of reaction condition such as reaction time, reaction temperature, temperature gradient, flow rate and pressure, thus enabling stable reaction process.